There is no kernel-xen-source.
xen ist just a configuration option of kernel-source. You will find the used
configuration in /usr/src/linux/arch/i386/defconfig.xen ( for 32bit kernels)
or /usr/src/linux/arch/x86_64/defconfig.xen ( for 64bit kernels )

That's true of all flavors except for -xen and -rt. Those two flavors
have additional patches that aren't installed as part of kernel-source.

I'm writing up a different email to explain how to get the kernel source
for the xen flavor.